The Environmental Protection Agency (EPA) establishes standards against which the relative abundance of compounds in samples may be compared and thus the compound may be determined. For example, through the use of standard compounds such as bromoflurobenzene (BFB) and decaflurotriphenylphosphine (DFTPP) volatile and semi-volatile organic compounds in water may be analyzed. The standard compounds are injected into a gas chromatograph-mass spectrometer (GC-MS) system under standard conditions. The resulting spectra are examined to determine the performance include ion mass resolution, relative ion abundance and mass accuracy over the range of ion masses analyzed. The mass spectrometer is tuned to obtain a spectrum for BFB or DFTPP that meets EPA standards. Commercial mass spectrometers must be capable of being tuned to meet these EPA standards in order to accurately determine the presence of specific compounds in a sample.
In the analysis of air samples and other gaseous matrices, a popular method of collecting any volatile organic compound (VOC) present in the sample is to pump the sample, at a fixed rate, through a tube packed with a suitable adsorbent material. This is known as pumped sampling. An alternative method to pumped sampling is to allow VOCs in the air to migrate into the tube through natural diffusion. This is known as diffusive or passive sampling.
Once the VOCs have been collected, they are vaporized by heating the tube in a thermal desorption instrument. A flow of an inert gas, such as Helium or Nitrogen, is applied to the tube to sweep the VOC vapor into a gas chromatographic (GC) column for separation and analysis. An additional trap is usually employed in the thermal desorption instrument to effect a pre-concentration of the VOC analytes prior to their injection into the GC. FIGS. 1 and 2 illustrate this technique.
However, if proper care is not taken in the process of handling, sampling, storing and analyzing the tubes, errors may be introduced into the analytical results because of sample leakage from the tube or ingress of compounds from the storage equipment. Unfortunately, there has been no mechanism to ascertain whether such leakage has occurred and so, the validity of the data must be assumed.
Thus, it would be advantageous to provide a method and apparatus for verifying the integrity of thermal desorption sampling tubes.